1. Field of the Invention
The present invention relates to a multi-level information recording/reproducing method and apparatus for recording or reproducing multi-level information.
2. Related Background Art
The optical memory industry is lately expanding with the development of read-only CD/DVDs, the add-on type using metal thin films or recording dyes, and, ultimately, the rewritable type using magneto-optical materials or phase transition materials. Also, optical memories now have a wider range of applications from consumer products to external memories of computers. Research and development are underway to enhance the density of recording capacity, and one of the findings is that employing a blue-violet light source (wavelength: 405 nm) rather than a red light source (wavelength: 650 nm) is an effective technique of reducing the size of a light spot for recording and reproducing information. Another technique found effective in reducing the light spot in size is to raise the numerical aperture of an object lens from 0.6 or 0.65 to 0.85. Meanwhile, multi-level recording/reproducing technologies have been proposed which achieve higher efficiency with the same light spot size as before.
For instance, the inventors of the present invention have proposed, in Japanese Patent Application Laid-open No. H05-128530 as a system of multi-level recording/reproducing technologies, a recording method of recording multi-level information on an information track of an optical information recording medium by using a combination of an information pit's width in the direction of the track and a shift amount in the track direction with respect to a light spot for reproduction of its information pit; and a reproducing method for reproducing multi-level information based on a correlation between a detection signal learned in advance and a detection signal obtained form the light spot in reproduction of the information pit recorded by the multi-level recording method.
Another example is a presentation delivered at ISOM 2003, an international academic conference for researches in the field of optical disks (“Write-once Disks for Multi-level Optical Recording”, Conference Papers Fr-Po-04). The presentation is about 8-level recording/reproduction using a blue-violet light source (405 nm) and an optical system with an NA of 0.65, and setting the width in a track direction of a region (hereinafter, referred to as “cell”), which is a virtually provided area on an optical disk having a track pitch of 0.46 μm and which stores one information pit, to 0.26 μm.
The selection of an information pit of 8 levels is carried by dividing a cell's width in the track direction into 16 equal parts (16 channel bits), for example, as shown in FIG. 12, with Level Zero being no information pit recorded, Level One corresponding to the width of 2 channel bits, Level Two corresponding to the width of 4 channel bits, Level Three corresponding to the width of 6 channel bits, Level Four corresponding to the width of 8 channel bits, Level Five corresponding to the width of 10 channel bits, Level Six corresponding to the width of 12 channel bits, and Level Seven corresponding to the width of 14 channel bits.
Information pits having the thus selected levels are recorded at random, a photodetector receives light reflected by the information pits to detect a reflected light amount, and a signal reproduced from the obtained multi-level information pits is sampled when the center of the light spot falls on the cell's widthwise center in the track direction. Then, the amplitude of the reproduced signal for each level is distributed as shown in FIG. 13.
Here, the amplitude of the reproduced signal is standardized such that the reproduced signal has an amplitude of “1” when Level Zero, meaning no information pits recorded, is read successively, and that the reproduced signal has an amplitude of “0” when Level Seven information pits are recorded in succession.
The value of the reproduced signal for each level fluctuates because an information pit in question is influenced by its preceding and following information pits (inter-symbol interference). When the amplitude distribution of the reproduced signal for one level overlaps with those of adjacent levels as shown in FIG. 13, it is impossible to detect them separately with a fixed threshold.
The presentation at ISOM 2003 has introduced, as an example of how to solve the above problem, a separate detection method including a step of reading and storing a signal that is reproduced from a pit string consisting of an information pit in question and its preceding and following information pits, all three of which have known values (learning), and a step of comparing the stored value with a signal reproduced from the actual information pit (checking a correlation). According to this method, the recording density is approximately 16 G bit/inch2.
There has been an attempt to achieve as high a recording density as approximately 30 G bit/inch2 by modifying the multi-level method delivered at ISOM 2003 such that the light spot is reduced in size with the use of a blue-violet light source (405 nm) and an optical system having an NA of 0.85. However, this causes more inter-symbol interference and more overlapping of the reproduced signal amplitude distribution between neighboring levels shown in FIG. 13, which can increase erroneous reproduction unless some measure is taken.